Electrolytic cells



Feb. l0, 1970 s. l.. cAuvlN, JR 3,494,851

ELEGTBOLYTIG CELLS Filed May 17, 1967 2 Sheets-Sheet 1 SIDNEY L. CAUVIN JR.

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l ELECTROLYTIC CELLS Filed May 17, 1967 2 Sheets-Sheet 2 FLEXIBLE BOTTOM CATHODE BOTTOM DEFLECTION VS. AGE

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ATTORNEY Unid safes Patent o 3,494,851 ELECTROLYTIC CELLS Sidney Louis Cauvin, Jr., New Orleans, La., assignor to Kaiser Aluminum & Chemical Corporation, Oakland, Calif., a corporation of Delaware Filed May 17, 1967, Ser. No. 639,139 Int. Cl. C22d 3/02 U.S. Cl. 204-243 2 Claims ABSTRACT OF THE DISCLOSURE This invention relates to an electrolytic cell wherein the bottom plate of the supporting cell structure is mounted within the cell for displacement by forces which build up within the cell thereby increasing the life of the cell lining.

BACKGROUND OF THE INVENTION In the production of aluminum by the conventional electrolytic process, the electrolytic cell comprises in general a steel shell having disposed therein a carbon lining. The bottom of the carbon lining along with a layer of electrolytically produced molten aluminum which collects thereon during operation serves as the cathode. One or more consumable carbon electrodes is disposed from the top of the cell and is immersed at its lower extremity into a layer of molten electrolyte which is disposed in the cell. In operation, the electrolyte or bath which is a mixture of alumina and cryolite, is charged to the cell, and an electric current is passed through the cell, from the anode to the cathode via the layer of molten electrolyte while oxygen collects at the anode. The current then leaves the cell via suitable collector bars positioned in the carbon cathode lining. A crust of solidified electrolyte and alumina forms on the surface of the bath, and this is usually covered over with additional alumina.

In the conventional electrolytic process, use has been made of two types of electrolytic cells, namely, that commonly referred to as a .prebake cell and that commonly referred to as Soderberg cell. With either cell, the reduction process involves precisely the same chemical reactions. The principal difference is one of structure. In the prebake cell the carbon anodes are prebaked before being installed in the cell, while in the Soderberg cell or selfbaking anode cell the anode is baked in situ, that is, it is baked during operation of the electrolytic cell, thereby utilizing part of the heat generated by the reduction process. The instant invention is applicable to either cell.

The conventional aluminum electrolytic cell carbon cathode consists of either a rammed monolithic carbon lining with suitable collector bars embedded therein or prebaked carbon blocks with suitable collector bars cast in, for example, with cast iron or cemented in with carbon or graphite paste. Several inches of thermal insulting material, usually alumina or bricks, is placed below the carbon. The carbon lining and insulation is encased in a steel container or shell which is usually reinforced to retard distortion. With these designs, the entire carbon mass conducts electricity and also acts as a container for the molten aluminum and molten electrolyte. Under normal operating conditions the electrolyte or constituents thereof penetrate into and through the carbon lining and into the insulation.

Cathode failure is caused by yforces generated in or beneath the carbon lining, which gradually fracture and buckle the lining allowing molten aluminum to attack and dissolve the steel collector bars. There are two popular theories on what causes these forces and how they act.

The rst and more traditional theory is that sodium from the bath penetrates into the cathode and reacts 3,494,851 Patented Feb. 10, 1970 ice with the carbon causing it to swell. This in turn causes the side walls to be pushed out and eventually the bottom to heave. 'Ihe theory is that after the electrolysis begins, sodium is released at the bath-metal pad interface, or formed by the reaction of aluminum with the bath, and dissolves in the metal pad and diffuses into the lining. At the same time, the lining slowly graphitizes by some unknow mechanism. A lluorine-containing species diffuses from the bath through the lining at a slower rate than the sodium. This slower rate may be attributed to the greater size of the molecule or lower vapor pressure. There it reacts with sodium forming sodium fluoride and other products. This may be followed by diffusion of bath into pores containing sodium and sodium fluoride. While this diffusion might be as a liquid, in a normal electrolytic cell for the production of aluminum where a carbon lining is underneath the metal pad, vapor diffusion seems more likely. Absorption of free sodium metal into the carbon lattice expands the lining. Because a reduction cell is enclosed in a steel shell, the expanded carbon lining buckles. This buckling is accompanied by formation and growth of cracks. These cracks lill with molten aluminum, which can eventually reach either the sides or the steel collector bars and cause failure.

The second and newer theory, which is rapidly being accepted by those skilled in the art, is that during normal operating conditions the bath constituents penetrate into and through the carbon lining into the alumina insulation. When the bath reaches an isotherm at which one of the constituents freezes out, this constituent will crystallize and form columnar crystals growing at right angles to the isotherm surface. Over a period of time, the forces generated by crystal growth gradually exceed the breaking strength of the carbon lining and the lining fractures and heaves. The whole process of movement down a ternperature gradient, lfreezing at a particular isotherm with the formation of columnar crystals causing an expansion perpendicular to the isothermal plane and stratilication and layering of the carbon is analogous to the frost heave which occurs beneath cold storage buildings or near the surface of the ground in cold weather. lIn the latter case, moisture can be drawn upward against the force of gravity to the 0 C. isotherm where it freezes as columnar crystals whose growth can produce tremendous forces, at right angles to the 0 C. isotherm, capable of lifting buildings. In both cases, characteristic layered structures or lenses of columnar crystals have been observed.

Prior art proposals have largely involved providing some procedure for the carbon current conducting lining to absorb these expansive forces either through expansion joints in the lining or other suitable means.

SUMMARY OF THE INVENTION It is an advantage of the instant invention that it produces an electrolytic cell shell design which employs a ilexible bottom which will deflect when subjected to loading conditions as found in the cathode system. The ilexible bottom -must be strong enough to support the weight of lining material above it with a minimum deflection, but weak enough to deilect as crystal growth proceeds and before the crystal growth forces reach such magnitude as to fracture the carbon liner. -v

It is a further advantage of the instant invention that it will prolong cathode life since cathode failure is caused by heaving and cracking of the carbon liner under the pressure of crystal growth and the instant invention reduces or substantially eliminates this upheaval force. Additional benefits and advantages of the instant invention are that it results in lower power loss in the cathode over its life span, improved cell efficiency, and better cell current distribution due to the greater carbon lining stability resulting from the use of this invention.

Accordingly, the instant invention relates to an electrolytic cell which comprises a supporting shell open at the top and bottom. A botom plate is mounted within the shell for displacement by the build up of forces within the cell. A layer of thermal insulating material is disposed within the shell on the plate. A current conducting lining is disposed within the shell on the layer of thermal insulating material and current collector bars are positioned in the current conducting lining and extend through the shell of the cell. Desirably, horizontally and inwardly extending flanges are attached to the lower edge of the shell and the bottom plate is slidably mounted on the flanges with the dimensions of the plate being less than the internal dimensions of the shell so that the plate is free to flex under forces produced in the cell. Normally, the thermal insulating material is a granulated material and thus suitable sealing means will be provided on the shell so as to prevent the thermal insulating material from leaking out of the shell around the bottom plate.

BRIEF DESCRIPTION OFhTHE DRAWINGS FIGURE 1 is a front elevation view, partly in section and with parts removed for purposes of clarity of an electrolytic cell embodying the instant invention.

FIGURE 2 is a front elevation view, partly in section of an electrolytic cell shell showing the shell and bottom plate in greater detail.

FIGURE 3 is a section of a top plan view of a bottom plate according to the instant invention.

FIGURE 4 is a chart showing the bottom deflection of an electrolytic cell embodying the principles of this invention with age.

FIGURE 5 is a chart showing the sidewall distortion of an electrolytic cell embodying the principles of this invention with age and the sidewall distortion of a conventional electrolytic cell with age.

DETAILED DESCRIPTION Referring now to the drawings in which the same reference numerals have been applied to corresponding parts and with particular reference to FIGURE 1, an electrolytic cell is shown. The cell 10 comprises a supporting shell 12 open at top and bottom. A bottom plate 14 is mounted within the shell 12 for displacement by build up of forces in the cell 10. In the embodiment shown in FIGURE l this is through the employment of horizontally and inwardly extending flanges 16 which are attached to the lower edge of the shell 12. The bottom plate 14 is slidably mounted on the flanges 16 with the dimensions of the plate 14 being less than the internal dimensions of shell 12 so that the plate 14 is free to expand and also to flex under the forces produced during operation of the cell. A layer of thermal insulating material or insulation 18 is disposed within shell 12 on plate 14. A current conducting lining 20 of a suitable material, such as carbon, is disposed within the shell 12 on the layer of insulation 18. As has been explained previously, suitable current collecting bars 22 are positioned in the current conducting lining 20 and extend outwardly through shell 12 to a suitable electrical connection not shown. The electrolytic cell 10 is mounted by means of mounts 24 on piers 26 so that space is provided below the cell 10 for the bottom plate 14 to deflect. The layer of molten aluminum which collects on the lining 20 during operation of the cell and serves as a cathode is indicated generally at 28. One or more consumable carbon electrodes or anodes 30 is disposed from the top of the cell 10 through a suitable current conducting means not shown and is immersed at its lower extremity into a layer of molten electrolyte indicated generally at 32 which is disposed in the cell 10. As has been explained previously, electrolyte 32 is a mixture of alumina and cryolite. A crust 34 of solidified electrolyte and alumina forms on the surface of the bath or electrolyte 32 and this is usually covered with additional alumina not shown. Suitable stiffeners 36 are attached to botom plate 14 so that the bottom plate 14 will be strong enough to support the weight of insulation 18, carbon lining 20, molten aluminum 28, electrolyte 32, and crust 34 above it with a minimum deflection, but weak enough to deflect as crystal growth proceeds and before the crystal growth forces reach such magnitude as to fracture the carbon lining 20.

In the embodiment shown in FIGURE 2, suitable sealing means such as an ore seal 38, shown here as an angular sheet of appropriate material fastened to shell 12 and overlying bottom plate 14, are provided on shell 12 so as to prevent the thermal insulation 18 from leaking out of the shell 12 around bottom plate 14. This, of course, is highly desirable `when the insulating material 18 is a granulated material.

FIGURE 3 provides a clearer presentation of stiffeners 36 and also shows how bottom plate 14 rests on flanges 16 thereby permitting bottom plate 14 to flex.

FIGURE 4 is a chart showing the bottom deflection of an electrolytic cell constructed in accordance with the principles of this invention, that is, having a flexible bottom, vs. age. As can be seen from this chart, the bottom plate 14 is indeed bending downward as the pressure produced by the crystal lenses at the carbon-insulation interface increases.

FIGURE 5 shows the correspondingly low sidewall distortion of the electrolytic cell embodying the principles of this invention. As can be seen from FIGURE 5, sidewall distortion increased fairly constantly for the first 400 days and then appeared to be controlled by the bottom deflection and increased more slowly thereafter. These data contrast rather vividly with that shown in FIGURE 5 for the sidewall distortion of a conventional cell which can be seen to increase very rapidly with age. In fact, the average age at three inch sidewall distortion of a conventional cell is between 600 days and 850 days depending upon whether a gas calcined anthracite paste or an electrically calcined anthracite paste was used to produce the carbon lining. At 800 days the electrolytic cell embodying the principles of this invention had less than one and one-half inch sidewall distortion.

Thus, it can be seen that the instant invention provides a novel flexible bottom plate for the shell of an electrolytic cell which will deflect when subjected to loading conditions as found in the cathode system. The flexible bottom is strong enough to support the weight of lining material above it with a minimum of deflection but weak enough to deflect as crystal growth proceeds and before the crystal growth forces reach such magnitude as to fracture the carbon lining. Because cathode failure is caused by heaving and cracking of the carbon liner under the pressure of crystal growth, this invention will prolong cathode life by reducing or eliminating the upheaval force. Some additional benefits of the instant invention are lower power loss in the cathode over its life span, improved cell efficiency, and better cell current distribution, all due to the greater stability of the cathode system realized by the practice of the invention.

While there has been shown and described hereinabove a possible embodiment of this invention, it is to be understood that the invention is not limited thereto and that various changes, alterations, and modifications can be made thereto without departing from the spirit and scope thereof as defined in the appended claims wherein what is claimed is:

1. An electrolytic cell which comprises:

(a) a supporting shell open at top and bottom having horizontally and inwardly extending flanges attached to the lower edges thereof;

(b) a bottom plate mounted within the shell for displacement by the build up of forces within the cell,

said bottom plate being slidable on the anges, with the dimensions of the plate being less than the internal dimensions of the shell so that the plate is free to flex under forces produced in the cell;

(c) a layer of thermal insulating material disposed within the shell and on the plate;

(d) a current conducting lining disposed within the shell and positioned on the layer of thermal insulating material; and

(e) current collector bars positioned in the current conducting lining and extending through the shell.

2. The electrolytic cell of claim 1 wherein the thermal insulating material is a granulated material and sealing means are provided on the shell so as to prevent the thermal insulating material from leaking out of the shell around the bottom plate.

References Cited UNITED STATES PATENTS JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner 

